Apparatus for determining the frequency of an electrical signal has long been known. A conventional apparatus for determining the frequency is usually referred to as a frequency counter and frequently includes a visual display showing in digital form the frequency measured. The apparatus typically counts the number of cycles of an incoming signal during a sampling period.
Frequency counters typically operate by counting voltage transitions on an electrical line across a zero amplitude threshold, where this electrical line carries the sum of all signal and noise sources contributing to an electrical input (for example, a radio antenna). If no single signal has sufficient strength to dominate the sum of all signal and noise sources on this electrical line, then a frequency determined in this manner will not correspond to the frequency of any individual signal. A frequency determination is only “valid” if a single signal is dominant to all other signals and noise present on the electrical line during the sampling period used to determine frequency.
To eliminate “false readings” of determined frequencies that do not correspond to the frequency of a real signal, prior art frequency counters have used several techniques to validate a frequency determination.
Some prior art frequency counters suppress a frequency report unless the same frequency is determined during a plurality of successive sampling periods. However, this method forces a delay in reporting the frequency of a new signal, for the duration of multiple sampling periods of sufficient length to determine the frequency individually. In addition, this method is not capable of reporting the frequency of a signal which is dominant for a shorter interval than multiple sampling periods, as is often true for weaker radio signals that achieve dominance only during very brief periods of optimal reception conditions.
Other prior art frequency counters suppress a frequency report unless signal strength measurements have indicated a sudden rise in total signal strength, presumably indicating the start of a radio signal of greater strength than all other sources combined. However, this method is not capable of reporting the frequency of a signal which is near the strength threshold for dominance, and is not capable of reporting the frequency of a signal which is continuously active. In addition, this method requires additional signal strength measurement apparatus.
Accordingly, there is a need for a frequency determination apparatus that verifies the presence of a dominant signal during the sampling period used to determine the frequency of this signal, and is sensitive to signals that are near the dominance threshold for signal strength, without dependence upon a sudden change in total signal strength.
Frequency agile radio receivers are available that allow a listener to monitor conversations on each of numerous fixed frequencies. These receivers may monitor fixed frequencies or channels, tuning one at a time, typically in response to programmed instructions. These receivers typically include a memory in which frequency information for tuning the receiver is stored. The receivers are usually programmable for altering or establishing the stored frequency information and monitoring process. The programming feature permits the stored frequency information to be changed, for example, in response to location changes or changes in frequency allocation information. However, when a frequency agile radio receiver is in operation in an area where there are many transient transmitters, for example, mobile transceivers, the user of the frequency agile radio receiver may not know the frequencies of transmission of the nearby transmitters. The typical receiver is incapable of determining the frequencies of these transmissions so the receiver cannot tune to monitor them. Since listeners desire to monitor these nearby transmissions, it is desirable to include, within the radio, apparatus for determining the channel including the frequency of these nearby transmissions and, preferably, to provide for manual or automatic tuning of the radio receiver to that channel for monitoring them.
Some prior art frequency counters for tuning radio receivers have been available in housings separate from the receivers and connected to the receivers by cables, making use awkward. Counting of the frequency and tuning of the associated receiver are disadvantageously slow because the special purpose counters require at least two complete frequency determinations before producing an output.
Some prior art radio receivers have included frequency counters within the same housing, but are incapable of concurrent scanning and counting operations. Some prior art radio receivers have allowed external devices, while the radio receiver is scanning, to write into the memory the frequency being read for scanning. An appropriately interfaced frequency counter, among many other conceivable sources, could be configured as such an external device for writing into this memory. However, these receivers have front-end electronics (including bandpass filters and pre-amplifiers) that are dedicated to the sole use of the receiver unit. In any configuration of such extant radio receiver and frequency counter devices, the frequency counter device would be required to provide a separate and independent front-end electrical system, including bandpass filters and pre-amplifiers.
Accordingly, there is a need for a frequency determining and radio tuning and receiving apparatus that can rapidly and accurately determine the frequency of a received radio frequency signal, that can store information identifying this frequency into a memory, that can access this memory to tune the radio receiver through a sequence of frequencies carrying communications of potential interest to the user, and that continue updating the memory with information identifying further determined frequencies while the radio receiver steps through frequencies identified by data stored in the memory.